In a communications voice network, a number of signalling tones are employed for communication between various devices. For example, DTMF (Dual Tone Multi Frequency) audio tone signals are used to carry dialed number information, and COT (Continuity Test Message) tones are used at the beginning of a call as part of the signalling protocol to check the continuity of the line. The network may also carry modem tones and fax tones.
As networks have increased in size and complexity, it has been necessary to introduce echo cancellation to maintain the quality of service that is perceived by the user. Echoes can arise in the voice system when part of the signal energy is reflected from impedance mismatches in the network. Those echoes often occur at a user terminal when they arise from the imperfect matching in 2 to 4 wire conversion transformers or hybrids. Those echoes have a detrimental effect on voice quality, particularly those having a delay greater than 15 milliseconds.
Echo cancellation is a particular problem when a traditional TDM (Time Division Multiplex) network is interfaced with a connectionless network, e.g. an asynchronous transfer mode (ATM) network or an Internet Protocol (IP) network providing long haul transport. Within the connectionless network, the voice traffic is transported in cells or packets. In such a system the use of echo cancellation is mandatory as echo signals even having a delay of less than 15 milliseconds may become noticeable as a result of packet delays in the network.
While the use of an echo canceller is effective and beneficial in suppressing echoes in voice transmission, its use can seriously impede the performance of data modems or distort the audio tone signals that are used for various signalling and control purposes. For example, distortion of COT tones can cause a call to be dropped, and distortion of DTMF tones can result in a failure to recognise digits that have been dialled by a user.
Further, while the echo cancellers in the network transmission equipment are effective in suppressing echoes in voice communications, they may have a detrimental effect to data communications that are sent over a voice network via a modem. Such modems are provided with their own echo cancellation facility. In order to avoid interaction between a modem's internal echo canceller and the network cancellers, modems transmit an echo canceller disabling tone which instructs the network to disable its echo canceller. This disabling tone is transmitted at the start of each modem transmission. The International Telecommunications Union (ITU) has developed the G.165 standard which gives recommendations on the operation and performance of electrical echo cancellers used in cancelling network echoes on international circuits or speech signals. G.165, in section 4, specifies the characteristics of an echo canceller tone disabler and specifies the disabling tone as a 2100 (+/−21) Hz tone with 180 (+/−)25 degrees phase reversals. A further standard ITU-T V.8 also specifies amplitude modulation of the 2100 Hz tone.
An additional problem with current networks is that of delivering a voicemail service, in such a service, voice messages are stored on a server which the user can access by dialling an appropriate number code. While the user is dialling the code to access the server, there is of course no voice traffic and thus, the echo canceller has no input information to perform the cancelling process The tones corresponding to the dialled digits are generally of too short a duration for the echo canceller to respond. This can result in dialling signals echoing around the loop resulting in incorrect double digit detection.
For the above reason it is necessary to detect the various types of tones so that the echo canceller can be selectively disabled as required. This has necessitated the use of separate detectors for each type of audio tone that is employed in the network. This requirement increases the cost and complexity of the network.